It is generally beneficial for electronic devices such as integrated circuits (IC) to function within expected operating modes, e.g., for which they were designed. For many electronic devices, it is important that they do not inadvertently enter an unexpected mode of operation. Certain voltage profiles, particularly at power-up, can cause an electronic device to enter an undesired mode of operation. In extreme cases, such operation may cause permanent damage to the device, for instance, through excessive current flow resultant from the operation at lower voltage.
Devices that contain re-programmable memory (such as Flash-based microcontrollers) and other particularly voltage sensitive components may enter an unintended mode where memory contents are altered so that proper future operation is disturbed or not possible. As it is generally desirable to provide a wide voltage range where operation is allowed, devices may require internal blocks or similar components to operate at supply voltage limits. For instance, such blocks may be needed where a voltage source provides very little margin for safe, guaranteed operation. If the device is operated outside of the specified range, erroneous operation may occur, perhaps even damage.
Exemplary voltage levels 100 relevant to this discussion are shown in FIG. 1, which reflects no particular scale. A minimum voltage V0 is required for operation of any components of a device. While the minimum voltage specified for operating a device is shown by voltage V3, a margin Mo exists below it wherein the device will continue to perform reliably. Below voltage V2 however, unreliable performance may be expected. Where the supply voltage to the device (e.g., Vcc, Vdd, etc.) is below V2 yet sufficient, e.g., above voltage V1, to perform certain minimal functions therein components of the device may function to keep the device in a reset mode.
Holding the device in a reset mode can thus deter improper operation. A reset threshold tolerance band Br thus exists between voltage levels V1 and V2. A margin Mr exists above V0 and below V1, above which the device can effectively be held in reset. In some modern microelectronic devices, for instance, microcontrollers, flash memories, and other such devices, the magnitude of the difference between such voltages, for instance even between V3 and V0, can be very small, e.g., on the order of fractions of a Volt or smaller.
One conventional approach to this issue is the use of a power-on-reset (POR) circuit to provide the reset functions. FIG. 2 depicts an exemplary POR circuit 200, typifying this conventional approach. Resistors 201 and 202 divide the supply voltage Vcc to provide a bias voltage Vg to the gate of transistor 203, e.g., effectively sensing the magnitude of Vcc. Transistor 203 operates with resistor 204 to provide an input to amplifier 205 based on the magnitude of Vcc. Amplifier 205 provides a reset signal R corresponding to this input, e.g., where Vcc is below V2 (and e.g., above V1).
POR circuits such as POR circuit 200 are common components in many modern electronic devices or systems. However, because their function entails reliable reset operation during low voltage conditions (e.g., below V2) and such precision is difficult to implement or achieve at such voltages, the reset functions they provide may lack precision, relative to other circuitry. For instance, some conventional POR circuits have tolerances that are on the order of ±20%.
Thus, where circuit 200 is implemented in an integrated circuit (IC), its trip point for generating its reset signal R may vary, e.g., from approximately 1.4 V to approximately 2.1 V over normal process and temperature (PT) variations. Such a degree of POR circuit imprecision has the undesirable consequence of restricting the voltage range in which the IC or other device is operated and various approaches have been used to attempt to remedy this.
One conventional approach to POR circuit imprecision relies on external components performing a supervisory function (e.g., supervisory components) to monitor system voltage and provide reset signaling where insufficient voltage is present, for instance when low voltage conditions occur (e.g., voltage is below V2). However, such supervisory components typically use special sensing circuits, some implemented using costly technology, to achieve precision monitoring. Further, the external supervisory circuits consume space and conductor availability on the circuit board housing them (and e.g., the IC with the POR circuit), which may be at a premium, and require electrical connections that can introduce negative heating and reliability effects.
Integrating voltage sensing precision into a device that itself may be inherently low-precision can sometimes reduce overall system costs. Thus, another conventional approach uses any of various trimming or tuning methods to achieve acceptable voltage monitoring circuit precision. However, this approach can be expensive in terms of manufacturing costs, because special circuits may be needed to implement it. The approach can also be expensive in terms of processing because, for instance in ICs, processing resources are typically demanded to provide the trimming/tuning functionality.
Moreover, the trimming or tuning may occur in some devices only once they have powered up, such as where trim and/or tune values, stored for instance in the device non-volatile memory, are used to facilitate the trimming/tuning. While such tuning may indeed bring a part's reset threshold detection to the desired precision, during the time before the precision tuning has been applied, the device remains exposed to potentially deleterious voltage. Under low voltage conditions or with an unfavorable voltage supply profile, retrieving and/or implementing the needed calibration settings may not be by entirely reliable; it may for instance be at risk of error.
Yet another conventional approach uses a relatively imprecise power-on reset circuit along with a more precise voltage monitoring reset circuit. In one such implementation, a voltage comparator with a reference voltage source is used. Using such an approach, the uncertainty in the trip threshold can be reduced as low as approximately five percent. Without special trimming methods however, it can be difficult to place the low voltage reset (e.g., trip) levels of such a device to allow both reset-free operation over an intended voltage range, and yet assert reset before faulty operation occurs, when voltage leaves the allowed voltage range. Further, conventional retrieval and implementation of reset calibration settings can be subject to error in the presence of unfavorable voltage supply profiles.